Most of resonant-mode power supplies are not capable to provide proper stabilization of voltages, currents or output power over the entire load range, i.e. from the open to shorted output. In general, any of these states, or both of them, can be regarded as the most adverse operating conditions. To remedy this problem, resonant power circuit configurations are modified by means of e.g. providing additional energy recirculation circuits that enable feeding energy from the resonant circuit back to the supply or employing advanced circuits for controlling switches.
The Polish patent application P-349476 discloses a method for the output voltage and current control in power supplies provided with circuits feeding excess energy from the resonant circuit series capacitor back to the inverter electric source (energy recirculation). The inverter has an output voltage limiter, the input of which is connected in parallel to a load, for example by means of a transformer playing also the role of the output transformer. The rectifier output of the quality-factor limiter is connected to the inverter DC power supply buses. The voltage limiter and the energy recirculation circuit operate in a mutually complementary manner, such that the energy recirculation circuit limits the resonance circuit current and, as a consequence, the excess stored energy is supplied back to the source from both the voltage limiter and the energy recirculation circuits. Since the power supply provides continuous energy recirculation to the source, the resonant circuit current waveform maintains a quasi-sinusoidal nature, even under no-load conditions, and the output DC or AC voltage magnitude is limited to the level set by the voltage limiter parameters. In another exemplary embodiment, a capacitor is connected to the power supply DC output in order to enforce quasi-sinusoidal currents in the resonant circuit. The capacitor is charged by a series of rectified sinusoidal voltage pulses until the capacitor voltage attains the switch-off level set by the input voltage divider and the reference voltage applied to the second input of the comparator under the condition that duration of the shortest pulse series equals three half-cycles of the resonant circuit self oscillations and pulse series are switched on and off at the instants in which switches' currents are approaching zero. Whereas during pauses between groups of pulses the resonant circuit is short-circuited by means of a switches adjacent to one pole of DC supply.
From the US patent application US2010/00205695 there is known a resonant converter having a system for adaptive control of dead time between pulses in order to improve the efficiency of the converter, reduce voltage and current stresses in power components and mitigate electromagnetic disturbances. A dead time between pulses generated by the switching circuit is adaptively set by a control circuit in conformity with the magnitude of the input voltage and controlled according to the current passing through the inductive element of the resonant circuit. The dead time may also be computed on the cycle-by-cycle basis from the current value or taken from a look-up table that sets the dead time in conformity with the input voltage and inductor current values.
From the US patent application US20030231514 there is known a series-parallel resonance converter and a method of operating such a converter, dedicated for high-voltage applications of the order of 100 kilovolts and more. The control circuit has two states of operation: the first state for controlling the system output parameters and the start state. In this embodiment of the invention the start of the conduction of the switches is synchronized with the value of the parallel resonant circuit current. More particularly, the start of the first conduction of one of the switches is carried out at the maximum parallel resonant circuit current and at the same polarity as in the series resonant circuit.
From the US patent application US20090034298 there is known a method for resonant-type AC-DC power supplies control with low power losses at low loading conditions and low standby power loss. The method is based on the frequency response and loading condition of the resonant circuit of a resonant converter, adjusts the switching frequency and the switching duty cycle so as to obtain a stable output voltage. The method also improves its performance employing zero voltage switching (ZVS) and output synchronous rectifiers. The resonant-type power supply controller utilizes a hybrid technique that consists in combining frequency modulation and pulse width modulation, referred to as frequency modulation hybrid pulse width modulation (FMHYPWM). The controller can also be used for power factor correction and to the output synchronous rectifier control.
Furthermore, in the Polish patent Application P-389886 there is described a method for controlling H-bridge in a resonant-type converter that consist in alternate switching of the bridge switches so that between turning-on pairs of switches: the first and third or the second and fourth, are alternately turned-on pairs of high-side switches: the first and fourth or low-side switches: the second and third.
A U.S. Pat. No. 6,151,231 discloses a series-resonant power converter featuring a tap winding on a transformer that is coupled to a resonant capacitor in a resonant tank circuit. The power converter comprises an assembly of switches, between which a resonant circuit with an output load is connected and a controller configured to stabilize the output voltages by controlling switching of the switches in response to a slow-response monitoring circuit configured to monitor the output voltage. It comprises an energy recirculation circuit, configured to clamp the voltage across the resonant capacitor when the control circuit detects light load conditions and otherwise to maintain the voltage across the resonant capacitor unaffected when the control circuit determines heavy load conditions.
Although all the above methods are useful, they do not take full advantages offered by the control utilizing self-oscillations of the resonant circuit.
The aim of the invention is to provide a method of controlling a resonant-mode soft-switched power supply capable to provide proper stabilization of the output voltage, current or power over the entire load range, i.e. from the open to shorted output.